Can someone run me through how to do this, my book is not helping at all

that being said i have gave it a go but would be helpful if someone could run through this practise question with me

1. A company manufactures widgets and knows that when they set the price of
one widget to £25 then their customers demand 30 widgets. However, if they
reduce the price of a widget to £20, demand then increases to 35 widgets. In
order to produce widgets the company faces fixed costs of £300 and variable
costs of £15 per unit.
a) Use the data regarding price and demand to determine the linear
demand function (recall the demand function will be of the form
p = aQ + b where p is price and Q is quantity) (9)